Increased interest in global environmental issues has led to a demand for superior fuel consumption performance in pneumatic tires for high-performance vehicles, along with steering stability and braking performance when traveling on wet road surfaces. As a result, by compounding silica in rubber compositions that form tread portions, dynamic visco-elasticity characteristics of the tread rubber such as loss tangent (tan δ) and the like has been improved, heat build-up has been suppressed, rolling resistance reduced, and fuel consumption performance improved, in addition to improving wet performance. However, silica has poor affinity with diene rubber and dispersibility tends to be insufficient. Particularly, when the particle diameter of the silica is small, dispersibility worsens and, as a result, the effects of achieving reduced heat build-up and improving wet performance have not been obtainable. Additionally, its reinforcement action is small compared to carbon black, and it has not been possible to fully assure wear resistance.
To resolve this problem, Japanese Unexamined Patent Application Publication No. 2009-91498A proposes improving the dispersibility of silica by a rubber composition in which silica is compounded in a terminal-modified solution polymerization styrene butadiene rubber where the terminals are modified by a polyorganosiloxane or the like, thereby reducing heat build-up (tan δ at 60° C.), enhancing wet grip performance (tan δ at 0° C.) and improving wear resistance. Additionally, Japanese Unexamined Patent Application Publication No. 2007-321046A proposes a rubber composition comprising a styrene-butadiene copolymer rubber and, per 100 parts by weight thereof, from 80 to 180 parts by weight of a filler comprising not less than 50 parts by weight of a silica, and from 5 to 60 parts by weight of a resin having a softening point of 100 to 150° C.